When an asteroid or
comet impacts a planetary body, it releases a tremendous amount of energy.
Except for objects smaller than a few meters, the impacting asteroid or comet is
obliterated by the energy of the impact. The impactor material is mixed
with the target material (the rock on the planet's surface) and dispersed in the
form of vapor, melt, and rock fragments.

During the impact,
sulfur in the impactor or in sulfur-containing target rocks can be injected into
the atmosphere in a vapor-rich impact plume. In some impact events, such
as Chicxulub,
the rocks hit by the impactor contain sulfur. Sedimentary rocks hit by an
impactor sometimes include large amounts of evaporites. Evaporites
are rocks that are formed with minerals that precipitated from evaporating
water, such as halite (rock salt) and calcite (calcium carbonate). Two
other very common evaporite minerals are gypsum (CaSO4 +
H20) and anhydrite (CaSO4), both of which contain sulfur
(S).

Projectiles also
contain sulfur-bearing minerals, particularly the mineral troilite (FeS), which
is obliterated in an impact event. This material releases its sulfur, which is
then injected into the stratosphere. The amount of sulfur injected into
the stratosphere depends partly on the composition of the projectile, which can
vary from one crater to another. Using chemical traces of the projectiles
left at impact craters, scientists can determine the type of meteoritic material
involved. Using this data, scientists can then calculate the amount of
sulfur each specific impact injects into the stratosphere. The amount of
this sulfur can be substantial, because meteoritic materials contain up to 6.25
weight percent sulfur. Consequently, even if the asteroid or comet does
not hit a S-rich target, it can still cause dramatic increases in the total
amount of atmospheric sulfur.

Once vaporized, this sulfur can
react with water to form sulfate (or sulfuric acid) particles. These
particles can greatly reduce the amount of sunlight that penetrates to the
surface of the earth for a period of up to several years. Over time, the
sulfate will settle out of the stratosphere (upper atmosphere) into the
troposphere (lower atmosphere) where they can form acid rain which can have
additional environmental and biological effects.

The table below shows
calculations of the abundances of sulfur added to the atmosphere during known
large impact events. These calculations are based on the amount of sulfur
in the projectile only, and do not take into account the sulfur present in the
target rocks.

ddir Projectile types: The majority of geological information about asteroids comes from
meteorites, which are their associated rock-type fragments. Meteorites, and
asteroids by association, are classified based on their chemical
composition.

Given below are descriptions of the various
meteorite types:

Ordinary Chondrites (H,L,LL):These stony meteorites are the most common
meteorites.They are
composed mostly of silicate minerals (olivine, pyroxene, plagioclase) and
represent undifferentiated primitive material from the solar nebula, dating back
over 4.5 billion years. Chondrites are characterized by small, globular,
millimeter-sized inclusions called chondrules. If you could remove
chondrules from the meteorites, they would roll across a table like a marble.
These meteorites also contain several percent metal. Both the chondrules and the
metal content of chondrites can be seen in the photo below. The sulfur in
chondrites is primarily in the form of troilite (FeS) - a sulfide
mineral.

(Above) The ordinary chondrite - Dos
Cabezas.

The S-type asteroid 243 Ida
(NASA).While
the link between specific types of meteorites and asteroids is uncertain, some
scientists have suggested that S- type asteroids like Ida are composed of
ordinary chondrite material.

Irons:Iron
meteorites are composed of a nickel-iron alloy along with trace amounts of
non-metallic minerals and sulfides. Some iron meteorites are thought to be
fragments of the iron core of a differentiated asteroid. The sulfur in
iron meteorites is primarily in the form of troilite (FeS) - a sulfide
mineral.

(Above) The iron meteorite -
Bagdad.

Shape model rendering from radar data
of the M-type asteroid 216 Kleopatra (NASA/JPL).The refelectance characteristics of
M-type asteroids like Kleopatra suggestthat they may be composed of iron-nickel which hints
at a possible source for iron meteorites.

Carbonaceous Chondrite (CI, CM, CV, CO, CK, and
CR):Carbonaceous
chondrites are very rare and primitive meteorites.These meteorites contain organic compounds as well
as hydrous silicates (water bearing minerals). The sulfur in carbonaceous
chondrites can take the form of sulfide minerals such as troilite (FeS),
elemental sulfur, or water soluble sulfate. The Allende meteorite (CV) shown
below is approximately 2.1% sulfur by mass, while CI carbonaceous chondrites
have ~6.25% sulfur.

(Above) The carbonaceous chondrite -
Allende.

The C-type asteroid 253 Mathilde
(NASA/JHUAPL).

While the link between specific types
of meteorites and asteroids is uncertain, some scientists have suggested that C-
type asteroids like Mathilde are composed of carbonaceous chondrite
material.

Stony Iron -
Mesosiderite:This
is an unusual type of meteorite that is composed of nearly equal amounts of
metals and silicates. Breccia is a rock type that contains broken rock fragments
welded into a finer grained matrix.
Mesosiderites probably represent the shattered regolith of an asteroid that has
been the target of several asteroid-asteroid collisions. Pieces of this regolith
can be blasted off the surface of a larger body, and eventually reach the Earth
as meteorites, or if large enough, as impacting bolides. The sulfur in
mesosiderites is primarily in the form of troilite (FeS) - a sulfide
mineral.

(Above) The mesosiderite - Clover
Springs

The S-type asteroid 951 Gaspra
(NASA).While
the link between specific types of meteorites and asteroids is uncertain, some
scientists have suggested that S- type asteroids like Gaspra are composed of
stony-iron meteoritic material, possibly including mesosiderite
material.

What does
enhancement mean?

The enhancement value listed in the table
is a calculation of how many times greater the overall sulfur content of the
stratosphere would be following a large impact event. The baseline for this
calculation is the background sulfur present in our current atmosphere.
This number likely fluctuated to a small degree over geologic time, especially
following periods of extreme volcanism.

What does Ir in
ejecta mean?

This is the amount of the rare trace
element iridium (Ir), a platinum-group mineral, sampled in an impact crater's
ejecta. The importance of iridium in impact ejecta comes from its very low
concentration in the Earth's rocks, and its relatively high concentration is
meteorites, comets, and asteroids. Anomalously high levels of iridium in a
thin clay layer at the Cretaceous-Tertiary boundary are what led Luis Alvarez,
(a Nobel Prize-winning physicist) and his son Walter (a geologist), to propose
the impact hypothesis for the K-T
mass extinction.

What are the Eltanin and Australasian impacts events? Do they have
craters associated with them?

The Eltanin event occurred over two million
years ago when a 1-4 km asteroid impacted in the Southern Ocean between the
southernmost tip of South America and Antartica. The impact evidence for Eltanin
stems from iridium anomalies in ocean drilling cores (see above). Although the
impact was very large, no submarine crater has been found.

The Australasian impact is inferred from
the huge number of tektites found over thousands of kilometers of southeast Asia
and Australia. Tektites are small teardrop or button-shaped rocks that are
formed by the solidification of molten droplets. The droplets were terrestrial
rocks and dirt that were superheated during the impact, ejected from their
source crater, and then rained down on the land and sea. The source crater
for the 700,000 year-old Australasian tektite strewn field has not been
found.

This web site is based on information originally created for the NASA/UA Space Imagery Center’s Impact Cratering Series.Concept
and content by David A. Kring and Jake Bailey.Design, graphics, and images by Jake Bailey and David A.
Kring.Any use of the information and images requires permission of the Space Imagery Center and/or David A. Kring (now at LPI).